1. Field of the Invention
The present invention relates to the compacting of potash salt for the production of fertilizer granulates, and in particular of potassium chloride. These potash salts were obtained by treating raw mixtures or single salts by way of crystallization or flotation.
2. The Prior Art
Potash salt granulates are usually produced by build-up and press granulation. In the build-up granulation process, moist salt from the dehydration stage is mixed with dry salt of the finest type (re-circulated dust) as well as auxiliary granulating agents, and adjusted with water or off-lye to the required granulating moisture. The salt is subsequently compacted in drums to produce round granules. The xe2x80x9cgreenxe2x80x9d granules so obtained are subsequently dried.
In the press granulating process, fine product that has already been dried is pressed on presses (e.g. roll presses) into the shape of so-called xe2x80x9cbarnaclesxe2x80x9d or shells with a thickness of a few millimeters. These shells are thereafter subjected to a grinding and screening process. The coarse screening product is the finished product, whereas the undersized grain is re-circulated (Ullmann""s Encyclopedia of Technical Chemistry, 4th edition, volume 13, page 471; Verlag Chemie, Weinheim). No auxiliary granulating agents are added during compacting. The resulting pressed granulates are solid and have a stable shelf life at the normal medium humidity values which are found at median latitudes.
At high relative air humidity values which are found, for example in tropical countries, it is found that the pressed granulate exhibits increased water absorption. This may furthermore lead to decomposition of the grain, to an increase in the amount of the fine component, and to hardening of the product.
Decomposition phenomena which may be occurring, for example in connection with salt lick stones for animal fodder can occur under the influence of the weather outdoors. These problems are dealt with according to U.S. Pat. No. 2,599,436, among others, as follows. The salt component, which is sodium chloride, as a rule, is mixed with components of up to 6% MgO as well as up to 12.5% of an about 20% MgCl2 solution, and then hardened in molds.
Such a procedure is not possible in connection with the press granulation of potash fertilizer salts because such compacting is a dry process in which the compacting pressure plays the decisive role for the strength of the granulates. Furthermore, the required chemical quality of the fertilizer granulates would be impaired in an impermissible manner.
It is an object of the present invention to produce a pressed granulate which is solid and stable also under extreme climatic conditions such as high relative air humidity values and elevated outside temperatures, and which will not decompose even when stored over longer periods of time. At the same time, the required chemical quality of the salt to be granulated is not to be impaired.
It has now been found that the water absorption of a pressed granulate, for example of a 60-type potash salt fertilizer, from the ambient moist air can be suppressed. This suppression occurs if small amounts of magnesium oxide in the form of powder are added to the dry fine salt to be granulated. A dry mixture is then produced, which is then processed into granules according to known compacting methods.
It has been found, furthermore, that the compacted granulate produced in this way has a final bursting strength even under elevated air humidity values and temperatures that is a higher bursting strength than the one of a comparably pressed granulate not treated as defined by the invention. The effect can be observed also after longer storage periods at high relative humidities. The product remains granular and stable.
The positive effect can be observed in connection with all commercially available MgO-specifications.
In a preferred embodiment of the process of the invention, fine salt to be granulated such as a fine, dry KCl-crystallizate is obtained in a dissolution process or a flotation process. It is mixed with finely ground magnesium oxide, and the mixture is then pressed into the shape of barnacles (or shells) with application of the usual compacting pressures, for example on a roll press. These barnacles are subsequently ground and processed into granulate with the required grain size by the usual circulation processing procedure including classifying and separating. According to the invention, contents of 0.2 to 2 percent, by weight, preferably 1 to 1.5 percent by weight of dry MgO powder based on the dry amount of granulate are metered into the granulate. Higher contents lead to an insignificant improvement in the result which, however, bears no comparison with the attendant reduction in the content of valuable material in the final granulate.
The positive effect of the invention is developed in the same way if prior to the pressing step, preferably burnt lime in the form of powder, or burnt dolomite (CaO/MgO) are metered into the calcium oxide components. They are present in small amounts according to the invention. The process is not limited to the compacting of potassium chloride, but applicable for other potash salt fertilizers as well, for example also for potassium sulfate.